Dark Ultra-Light Scalars and Cosmic Parity Violation

If the dark sector of the Universe consists of ultra-light scalars, their coupling to photon via a Chern-Simons term would induce a rotation of the polarization plane of the cosmic microwave background (CMB). This rotation would convert E -mode polarization into B -mode polarization, resulting in new CMB BB correlation and parity-violating TB and EB cross correlations. We review the subject giving details about the derivation of the rotational effects and summarizing the possible signals in current and future CMB B-mode experiments.